Viscosity measurements of ionomer solutions under high pressure were made using a high pressure rolling-ball viscometer(S. Sawamura et al., Rev. Sci. Instrum., 61, 871(1990)). In these experiments, Nafion(Du Pont Inc.)was used as an ionomer, and 99.5 Vol % ethanol(Nacalai tesque Inc.)was used as a polar organic solvent. The 8 kinds of different concentration solutions were prepared. The measurements were carried out at 25.0 ﾟC and up to 300 MPa. Polyelectrolyte effect, which was well known in ordinary polyelectrolyte solutions of polar organic solvent, was observed in these ionomer solutions under atmospheric pressure. Namely, the viscosity of the dilute solution increased with decreasing concentration of the sample solutions. The polyelectrolyte effect was also observed under high pressure. However, the effect was suppressed with increasing pressure. The similar behavior, . that is, the suppression of the- polyelectrolyte effect with adding a small amount of simple-salt was reported already by other workers in the other ionomer solutions under atmospheric pressure. But the pressure dependence of the viscosity of the ionomer solution and the suppression of the polyelectrolyte effect with increasing pressure were first observed in this study.Since the experiments have just finished, it is difficult to describe the considerations of the results of this research. But the followings were confirmed : (1)Polyelectrolyte effect was observed under high pressure, (2)The polyelectrolyte effect was repressed with increasing pressure. The repression observed here may be explained by the shape change of the polymer ions in the dilute solution. And the increment of dielectric constant of the solvent with increasing pressure may play important role for the repression of the polyelectrolyte effect.